Magnetic tapes have found various applications in audio tapes, videotapes, data backup tapes for computers, etc. In particular, in the field of magnetic tapes for data-backup (or backup tapes), tapes having memory capacities of several tens GB or more per one reel are commercialized in association with increased capacities of hard discs for back-up. Therefore, it is inevitable to increase the capacity of this type of a tape for data-backup so as to correspond to a further increased capacity of a hard disc. It is also necessary to increase the feeding speed of tape and a relative speed between the tape and heads in order to quicken the access speed and the data transfer speed.
To increase the capacity of tape for data-backup per one reel, the following are necessary: (1) the length of a tape per reel is increased by decreasing the total thickness of the tape; (2) the thickness demagnetization is decreased to shorten the recording wavelength by forming a magnetic layer with a thickness as very thin as 0.3 μm or less; and (3) the recording density in the tape widthwise direction is increased by narrowing the widths of the tracks to 15 μm or less.
When the thickness of the magnetic layer is reduced to 0.3 μm or less, the durability of the tape tend to lower. Therefore, at least one primer layer is provided between a nonmagnetic support and the magnetic layer. When the recording wavelength is shortened, the influence of spacing between the magnetic layer and the magnetic heads becomes serious. Thus, if the magnetic layer has large projections or dents, an output decreases due to spacing loss, and thus an error rate increases.
When the magnetic layer is formed with a thickness so thin as 0.3 μm or less and concurrently the recording wavelength is decreased, magnetic flux leakage from the magnetic recording medium is decreased. Therefore, it is preferable to use reproducing heads which make use of megnetoresistance elements capable of achieving high output from very small magnetic fluxes (hereinafter, referred to as MR heads).
When the recording density in the tape-widthwise direction is increased by narrowing the width of the tracks (the width of data tracks on which signals are recorded) to 15 μm or less, reproduction output decreases due to off-track. To overcome such a problem, track servo becomes necessary.
One of such track servo systems is an optical track servo system, in which pits for optical servo are formed by irradiation with laser beams or by depression with a stamper, and such pits are optically detected for servo tracking.
As other optical track servo systems of this type, JP-A-03-141087 discloses the formation of pits for optical servo on the magnetic layer of a floptical disc (an optical servo track type floppy disc), and JP-A-11-339254 and JP-A-11-213384 disclose the formation of pits for optical servo are formed on the backcoat layer of a magnetic tape.
In the optical track servo systems in which pits for optical servo are formed on the backcoat layer, the track servo is performed by detecting a difference in reflectance between the pits and the flat portion of the backcoat layer. In particular, when the backcoat layer having such pits is irradiated with light, light randomly reflects on the pits, and therefore, the intensity of reflected light which enters an optical detector is low. On the other hand, light regularly reflects on the flat portion, and thus, the intensity of reflecting light is high. This system makes use of such a difference to trace the servo tracks formed as the pits. Specifically, interlocking with the servo tracking on the backcoat layer, the magnetic head which records or reproduces signals on or from the magnetic layer is moved to perform servo tracing on magnetically recording tracks.
According to this system, if the pits for optical servo are formed by irradiation with conventional laser beams, the intensity of light randomly reflecting on the pits can be sufficiently lowered. However, the intensity of light which reflects on the flat portion of the backcoat layer of a conventional magnetic tape is low, and the reflectance on the flat portion largely fluctuates depending on a site of the magnetic tape. Thus, it is impossible to sufficiently increase the ratio of S/N of optical servo signals. The reason therefor is that keen attentions are paid to only the tape running performance on the backcoat layer of the a conventional magnetic tape, but not to the reflectance thereon.
In case where pits are formed in a backcoat layer by irradiation with laser beams or by depression with a stamper, the peripheries (or the edges) of the pits are inevitably raised, which causes the following problems. In case where the total thickness of a magnetic tape is 6 μm or less, the rigidity of the tape (i.e., ET3 in which E is a Young's modulus of a tape, and T is a total thickness of the tape) decreases, and therefore, it is needed to decrease the winding tension for the tape which is running. In this case, if the specific positions of the magnetic tape are raised as described above, the track-formed portion of the tape wound onto a reel becomes extremely high, which results in a disorder in the winding of the tape.
In addition, if the tape has the raised portions as described above, they are pressed against the side of the recording layer (magnetically recording surface) of the magnetic tape, so that the surface of the recording layer becomes uneven, which results in low reproduction output. In case of a magnetic disc employing the optical servo track system, such winging as is made on the tape is unnecessary, and therefore, such disorder in the winding or the pressing by the raised portions do not occur, even though the peripheries of the pits for optical servo are raised. That is, these problems are peculiar to the magnetic tapes. To solve these problems, it is desirable to decrease the height of the raised portions of the tape to not higher than the height of the maximum projection of the flat portion thereof.
When the pits are formed on the backcoat layer by irradiation with laser beams, the coating surface of the backcoat layer is baked off by the energy of laser beams so as to form a pattern of pits. This method provides higher productivity, however, has a problem in that the numerous particles of burnt residues as the result of the laser baking for forming a pattern, undesirably, adhere to the pits and their peripheries. If the burnt residues are left as they are, they cause not only contamination of the tape-running system but also decrease in the ratio of S/N of optically read signals on the backcoat layer and the dropping-out of the magnetic layer due to the adhesion of the burnt residues. Further, the reflectance on the flat portion of the backcoat layer decreases, so that the reflectance in the lengthwise direction of the tape largely fluctuates. This fluctuation also decreases the ratio of S/N of optically read signals. Therefore, the removal of such burnt residues is necessary.
It is known that burnt residues which remain after the formation of pits for optical servo by irradiation with laser beams are removed using solid CO2, which has been used for removing such burnt residues from a floptical disc having pits for optical servo formed thereon (U.S. Pat. No. 5,419,733). In case of a floptical disc, the surface area to be cleaned is limited, and the solid CO2 can easily be sprayed to clean the surface by rotating the disk a number of times at a high velocity.
If this method is applied to clean a magnetic tape, the total surface area of the lengthy tape to be cleaned is enormous, and the amount of solid CO2 blown onto the tape a lot of times becomes far larger as compared with the disc. Therefore, the cleaning efficiency is poor. The magnetic tape confronts a further problem from which the floptical disc has never suffered: that is, the burnt residues remaining after the formation of the servo pattern by irradiation with laser beams adhere and transfer when the magnetic tape is again wound, and such burnt residues, in turn, adhere to the drive guide roller and the magnetic heads.
Alternatively, the surface of the magnetic tape is cleaned, for example, by allowing a tissue cleaning tape to contact with the front and back surfaces of a magnetic tape. This method is unsatisfactory, because the cleaning of the flat portion of the backcoat layer is insufficient, and also the effect of cleaning the interiors of servo dots formed as pits by laser beams is poor. The above cleaning treatment in combination with a blade treatment is also possible. However, a strong blade treatment may damage the backcoat layer, since the backcoat layer has a lower strength than the magnetic layer. On the contrary, if a weak blading treatment is made on the backcoat layer, the burnt residues thereon cannot be removed. Thus, this method is unsuitable for large-scale production, because selection of the conditions for the cleaning is difficult. Still worse, this method has substantially no effect of cleaning the interiors of the servo dots formed as the pits.
The present invention has been completed to solve the foregoing problems of the prior art.